The United States strongly warned Iran on Wednesday against closing a vital Persian Gulf waterway that carries one-sixth of the world's oil supply, after Iran threatened to choke off traffic through the Strait of Hormuz if Washington imposes sanctions targeting the country's crude exports.

The increasingly heated exchange raises new tensions in a standoff that has the potential to spark military reprisals and spike oil prices to levels that could batter an already fragile global economy.

Iran's navy chief said Wednesday that it would be "very easy" for his country's forces to close the strategic Strait of Hormuz, the passage at the mouth of the Persian Gulf through which about 15 million barrels of oil pass daily. It was the second such warning by Iran in two days, reflecting Tehran's concern that the West is about to impose new sanctions that could hit the nation's biggest revenue source - oil.

"Iran has comprehensive control over the strategic waterway," Adm. Habibollah Sayyari told state-run Press TV, as the country was in the midst of a 10-day military drill near the strategic waterway.

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"This is not just an important issue for security and stability in the region, but is an economic lifeline for countries in the gulf, to include Iran," Pentagon press secretary George Little said. "Interference with the transit or passage of vessels through the Strait of Hormuz will not be tolerated."

Rebarich declined to say whether the U.S. force had adjusted its presence or readiness in the gulf in response to Iran's comments, but said the Navy "maintains a robust presence in the region to deter or counter destabilizing activities, while safeguarding the region's vital links to the international community."

Iran's threat to seal off the gulf, surrounded by oil-rich gulf states, reflects its concerns over the prospect that the Obama administration will impose sanctions over its nuclear program that would severely hit its oil revenue. Iran is the world's fourth-largest oil producer, pumping about 4 million barrels a day.

Gulf Arab nations appeared ready to at least ease market tensions. A senior Saudi Arabian oil official said gulf Arab nations are ready to step in to offset any potential loss of exports from Iran.